1. Field of the Invention
This invention generally relates to a terminal apparatus having a handset, and, in particular, to a terminal apparatus having a handset which includes a transmitter, a receiver and a hook switch and which may be connected to a main body of the terminal apparatus through a 4-pin modular jack.
2. Description of the Prior Art
Various kinds of communication terminal apparatuses for carrying out transmission and reception of data using a telephone network as a transmission line are currently in use, and such communication terminal apparatuses include facsimile machines, personal computers, and word processors. In such communication terminal apparatuses, in general, in order to obtain a reduction in the total cost, it is often so structured that a communication circuit of a telephone is provided in the main body of a terminal apparatus and a handset provided with a transmitter and a receiver is connected to the terminal apparatus main body through a modular jack.
A hook switch is typically provided in a telephone function, and when this hook switch is provided in the terminal apparatus main body, use is made of a 4-pin modular jack for transmitter and receiver as shown in FIG. 5 for a connection between the terminal apparatus main body and a handset. However, when a hook switch is provided in a handset, use is made of a 6-pin modular jack. FIG. 6 illustrates a connection example between the terminal apparatus main body and the handset in such a case. As shown, there is provided a receiver amplifier 1 in the terminal apparatus main body and an audio signal is input into the receiver amplifier 1 from a communication circuit which is not shown. The receiver amplifier 1 amplifies the thus input audio signal to a predetermined level and outputs the audio signal to a receiver 2 through a first pin indicated by circled 1 of a modular jack. A transmitter 3 is provided in a handset and it converts an audio signal into an electrical signal and outputs the audio signal to a transmitter amplifier 4 through a third pin indicated by circled 3 of the modular jack. The transmitter amplifier 4 amplifies the thus input audio signal to a predetermined level and outputs the amplified signal to the before-mentioned communication circuit.
Also provided is a hook switch 5 which is set open when the handset is placed in a predetermined position, such as a handset rest, i.e., during on-hook, and it is closed when the handset is lifted from the handset rest, i.e., during off-hook. An input to a buffer 6 pulled up through a resistor R is connected to the hook switch 5 through a fifth pin indicated by circled 5 of the modular jack, so that when the hook switch 5 is set open, the buffer 6 supplies an output at high level, whereas, when the hook switch 5 is closed, the buffer 6 supplies an output at low level. The buffer 6 is typically of the Schmidt trigger type, and it detects the on/off condition of the hook switch 5 and supplies its output to the before-mentioned communication circuit.
However, in such a prior art communication terminal apparatus with a handset, if the hook switch is disposed in the handset, use is made of a 6-pin modular jack as described above, so that there are following disadvantages. That is, for a communication apparatus, it is often desired that variations be provided in model using the same type of main body frame and a handset is made as an optional part. In such a case, as shown in FIGS. 7a and 7b, a handset rest 9 is made as a separate element which can be attached to a side of a main body frame 10, and a handset 11 is placed on the handset rest 9 to thereby define a handset assembly 12. In such a case, it is preferable to provide the handset assembly 12 to be detachable, so that it can be removed from the main body frame 10 when it is not expected to be used for some time or during shipping. However, if the handset assembly 12 is provided to be detachable, the hook switch is required to be provided in the handset 12. However, in a terminal apparatus provided with a communication function using a telephone network, in particular a facsimile machine, a 6-pin modular jack 7 or connection to an actual telephone is typically provided, as shown in FIG. 7a. However, as shown in FIG. 7b, if a 6-pin modular jack 8 is also provided in the main body 10 for the handset, the same terminal apparatus comes to be provided with two modular jacks same in structure but different in interface. As a result, with such a structure, there is a chance that an erroneous connection is established.
Under the circumstances, in order to obviate the disadvantages as described above, it is conceivable to establish connection between a handset and a terminal apparatus main body with four signal lines as shown in FIG. 8. It is to be noted that those elements identical to those shown in FIG. 6 are indicated by identical numerals. According to this method, the three return lines (ground lines) within the handset are substituted by a single common line. However, with this structure, since the return lines of all of the receiver 2, transmitter 3 and hook switch 5 are commonly connected, it is not advantageous because noise may be transferred between the receiver 2 and the transmitter 3. In addition, with this structure, the transmitter 3 cannot be constructed in the balanced type because, in the balanced type, two signal lines of the transmitter 3 are required to float relative to ground.